Three years from now, tablet computers will outsell traditional Windows PCs, and do so by a whopping 72%, according to the latest projections from Gartner. In between, PC shipments drop at ever faster rates.

Three years from now, tablet computers will outsell traditional Windows PCs, and do so by a whopping 72%, according to the latest projections from Gartner. In between, PC shipments drop at ever faster rates.

Some of that decline will be made up by the faster growth in "ultramobiles," the new breed of Windows 8 devices such as Microsoft Surface Pro. But the PC decline is permanent, reflecting a "long-term change in user behavior," according to the Gartner statement.

Most users "will be satisfied with the experience they get from a tablet as their main computing device," according to Carolina Milanesi, a research vice president at Gartner. "As consumers shift their time away from their PC to tablets and smartphones, they will no longer see their PC as a device that they need to replace on a regular basis."

The report, "Forecast: Devices by Operating System and User Type, Worldwide, 2010-2017, 1Q13 Update," is on Gartner's website.

Gartner predicts that the traditional PC notebooks and desktop market will decline 7.6% in 2013. If you factor in the growth of the ultramobiles, the decline is less: 3.5%. Total PC shipments for 2013 are forecast at 315 million units, compared to 341 million in 2012. Ultramobiles will account for 23.6 million in 2013, compared to just 9.8 million in 2012.

By contrast, Gartner predicts tablets shipments will reach 197 million units in 2013, a 69.8% increase over 116 million units in 2012. And that kind of growth will be visible in all markets, including so-called emerging markets in Asia, Africa and Latin America. Tablets are a far more likely mobile phone companion than a notebook PC for consumers, according to Gartner.

By the end of 2017, Gartner predicts tablet shipments of 468 million compared to 272 million PCs.

Mobile phones generally, which Gartner does not break down into feature phones and smartphones, will keep growing but not as fast: 1.9 billion units in 2013 to 2.1 billion by the end of 2017.

The change in device type reflects a change in operating system dominance. Gartner has Android currently in first place in terms of the number of devices on which it's running: 891 million, rising to 1.5 billion in 2017. Windows, in all its flavors combined, stays in second, most of its growth attributed to Windows 8 mobile devices and Windows Phone. Apple's iOS and OS X platforms are third, rising from 293 million units total this year, to 504 million in 2017, most on expected growth in its iPhone and iPad lines. BlackBerry (still labeled "RIM" in Gartner's chart), grows slightly this year, to 31 million units, but Gartner forecasts a future, to 24 million units in 2017.

In the shares of operating systems (OSs) in device sales, the shift to mobile and the fight for the third ecosystem becomes more evident; Android continues to be the dominant OS in the device market, buoyed by strong growth in the smartphone market. Competition for the second spot will be between Apple's iOS/Mac OS and Microsoft Windows.

Tablets are not the only device type that is seeing aggressive price erosion; smartphones are also becoming more affordable, driving adoption in emerging markets and the prepay segment in mature markets. Of the 1.875 billion mobile phones to be sold in 2013, 1 billion units will be smartphones, compared with 675 million units in 2012.

"The trend toward smartphones and tablets will have much wider implications than hardware displacement," said Milanesi. "Software and chipset architecture are also impacted by this shift as consumers embrace apps and personal cloud."